This invention relates to a positioning control system for a fluid powered cylinder and, more particularly, to a device for accurately detecting the linear displacement position of a piston rod and a device for effecting control on the basis of the detected data such that the cylinder will be stopped at a desired intermediate position in the cylinder stroke by a brake operation.
The operation of a conventional fluid powered cylinder is limited only to a full-stroke operation. It has recently been proposed in the art of pneumatic cylinders to stop the cylinder at a desired intermediate stroke position by employing a cylinder having a brake. An example of such prior art is shown in FIGS. 1 and 2. A cylinder 1 is provided with a brake 2. A brake cylinder 4 is actuated by changing over of a brake electromagnetic valve 3 and braking force is thereby exercised on a piston rod 5. Reference numeral 6 designates a direction control valve for switching the direction of driving the rod, 7 a pneumatic source, and 8, 9 throttle valves for controlling the drive speed. A detection section 10 for detecting a stroke position of the cylinder includes a roller 10a which is constantly in contact with the rod 5 and rotates following the linear movement of the rod 5 and an incremental encoder 10b (FIG. 2) which generates an incremental pulse in response to the rotation of this roller 10a. When the cylinder 5 is at the origin, a counter 12 is reset by an origin set switch 11 and incremental pulses provided by the encoder 10b thereafter are counted by the counter 12. The count value of the counter 12 represents the cylinder rod position. Set positions of various operations set by a setter 13 are compared with this count value in a comparison circuit 14. The comparison operation in the comparison circuit 14 and the operation of a drive circuit 16 are controlled by a sequence circuit 15. The output of the comparison circuit 14 is supplied to the drive circuit 16 whereby a drive signal for switching the direction control valve 6 and the brake valve 3 is supplied to these valves 3 and 6. For example, a desired stroke intermediate position is set by the setter 13 and when the present position of the rod 5 measured by the detection section 10 coincides with this set position, the brake valve 3 is switched to actuate the brake 2 and simultaneously the direction control valve 7 is switched to a neutral position to stop the rod 5.
Despite braking, the rod 5 does not stop instantly but an overrun takes place. In the prior art devices, not much consideration has been given to any countermeasure against overrun. In these devices, an only measure taken is to apply brake at a position which is before a desired stop position by a predetermined distance on the assumption that the amount of overrun is constant. In actuality, however, the amount of overrun differs depending upon the velocity of movement of the rod 5. In a transition period after the start of the movement of the rod 5, the amount of overrun differs also depending upon the acceleration of the movement of the rod 5. FIG. 3 shows an example of changes in velocity, acceleration and overrun amount against the amount of movement of the rod 5 with the lapse of time from the start of the movement. As will be understood from the figure, the velocity, acceleration and amount of overrun change instably in the transition period of rising despite the intended movement at a constant velocity. This instable period is, for example, of the order of 30 mm in the rising period.
In the prior art device, therefore, no brake control taking into account the amount of overrun corresponding to the velocity has been made and this has placed a limit upon the accuracy of the stop control. Further, a proper brake control is difficult in the transition period in the rising because the velocity, acceleration and overrun amount are instable and therefore cannot be estimated and hence the rod 5 cannot be stopped in this portion. This is not only because of the instability in the overrun amount but also because of the manner of starting the movement of the rod. Since in the prior art the release of the brake and the switching of the direction control valve 6 are simultaneously performed in the starting period, a rapid initial dashing of the rod 5 occurs due to an abrupt release of the brake in a state in which the drive force is being applied to the rod 5. This prevents a fine stop control in the rising period.
In a hydraulic cylinder which does not use a brake, overrun occurs similarly when the rod is to be stopped at a desired intermediate position in the cylinder stroke.
Furthermore, since in the rod position detection section 10 in the prior art device, displacement of the rod 5 is transmitted by friction between the roller 10a and the rod 5, slippage occurring between the roller 10a and the rod 5 makes an accurate detection of the position impossible and therefore adversely affects the accuracy in stopping of the rod 5.